The Fertile Window - Scientific Literature Review

The Fertile Window - Scientific Literature Review

Intercourse
timing and frequency relative to ovulation is possibly the most
important factor affecting the chances of conception. It is also the
element over which couples trying to conceive have the most degree of
control.

Couples who are aware of their fertile time and focus
intercourse during this time have been found to have much higher
conception rates than couples who are unaware of their most fertile
time (Hilgers et al 1992). Identifying the fertile window is thus
of great importance for couples trying to conceive.

Identifying
the fertile window also allows couples and their healthcare providers
to retrospectively determine if they have been timing intercourse
optimally and may help to decide whether diagnostic investigations
or interventions are needed (if intercourse has been optimally timed
over several cycles with no pregnancy) or whether they may be
postponed (if intercourse has not been timed optimally).

What is the fertile window?

The
fertile window is made up of the days in the menstrual cycle where
pregnancy is possible. The length of the fertile window has been
determined by documenting the farthest away from ovulation that
intercourse can result in conception. To determine the length of the
fertile window and the probability of pregnancy relative to ovulation
we need to know the maximum lifespan of the male sperm in the female
reproductive tract. While the human ovum has a lifespan that is
likely not more than 24 hours, making conception highly unlikely
after the day of ovulation, the lifespan of the sperm is potentially
longer and more variable.

The studies

A number
of studies have assessed the length of the fertile window and the
probability of pregnancy on different days of the menstrual cycle
relative to ovulation. To determine this fertile window and
day-specific pregnancy probabilities, investigators of several
studies asked couples to record each act of sexual intercourse during
the menstrual cycle. Ovulation was determined using a variety of
methods. The majority of these studies estimated ovulation using
basal body temperature (Barrett and Marshall 1969; Royston 1982;
Schwartz et al 1980; Columbo and Masarotto 2000), yet studies have
also been done using cervical fluid (World Health Organization 1983;
Hilgers et al 1992) or the measurement of urinary hormone levels
(Wilcox et al 1995) as the marker of ovulation.

Findings

Based on
two widely cited studies (details below) the maximum fertile window has been
determined to include the day of ovulation and the five days
before ovulation. The Barrett and Marshall (1969) study was based
on data from 241 British married couples who were charting basal
body temperatures. The Wilcox et al (1995) study relied on data from
221 healthy North Carolina women who were planning to become
pregnant.

The most
likely day for intercourse to result in conception according to the
1969 study was two days before ovulation. According to the 1995
study, the most likely day for intercourse to result in pregnancy was
on the day of ovulation itself. Further studies have been done,
however, to account for measurement error in both studies and to
exclude early pregnancy losses from the Wilcox et al data set. With
the corrected data, the most probable days for intercourse to result
in pregnancy have been found to be one or two days before ovulation
rather than the day of ovulation itself (Stanford et al 2002).

The
limited sample size of the studies, however, must be noted, though a
larger multi-center study from 881 women including 7017 menstrual
cycles (Colombo and Masarotto 2000) also suggests that intercourse on
the day prior to ovulation is most likely to result in pregnancy.

While the
results from the studies may be based on a small sample size and may
be potentially error prone, they show that intercourse on the few
days before ovulation and the day of ovulation itself are the
days most likely to result in conception. This has been confirmed by
a much larger study involving 119,398 cycles from women trying to
conceive which revealed that 94% of all conception cycles included
intercourse on at least one of the three days up to and including the
day of ovulation (FertilityFriend.com 2004). Additionally, all of
these studies found that multiple acts of intercourse within the
fertile window increased the probability of conception.

Conclusions and recommendations for those trying
to conceive:

The
combined results of these studies indicate the following:

Couples
trying to conceive should try to focus intercourse within the days
immediately preceding and including ovulation.

Intercourse
in the fertile time should not be limited unless there is a medical
reason to do so.

Diagnostic
investigations or treatment interventions might be premature if
there is prolonged unwanted non-conception yet intercourse has not
been focused on the most fertile days.

Couples
who are hoping for pregnancy should not anticipate positive results
or begin to test for pregnancy when intercourse has not occurred on
at least one of the most fertile days.

Additionally,
because the most fertile time begins at least a couple of days
before ovulation, couples using devices such as OPKs (ovulation
prediction kits) or fertility monitors should begin to have
intercourse prior to observing a positive or a peak so as not
to miss an opportunity to conceive (since these devices typically
only indicate a peak or positive on the day prior to ovulation).

The
results of these studies also highlight that learning to recognize
the most fertile days which occur prior to ovulation is of prime
importance for couples trying to conceive.

References

Barrett,
J., Marshall, John (1969). "The Risk of Conception on Different
Days of the Menstrual Cycle." Population Studies23(3):
455-461.

World
Health Organization (1983). "A prospective multicentre trial of
the ovulation method of natural family planning. III. Characteristics
of the menstrual cycle and of the fertile phase." Fertil
Steril40(6): 773-8.